This invention relates to the field of low tire warning systems for automotive vehicles and more specifically to a warning system that derives information for determining when any particular pneumatic tire is low by monitoring the angular rotation of each wheel and axle torque of the driven wheels.
Several low tire detection systems have been described in the literature, including those that monitor tire pressure and profile height. More recent systems have been described which utilize effective rolling radius calculations to determine when the radius of one of the wheels varies. The generally employed principle of using the effective rolling radius relies on the fact that a wheel with a flat or low pressure tire has an incrementally smaller effective rolling radius than a nominally inflated tire. Often, wheel displacement sensors are used to measure the angular displacement of each wheel. Each of these measurements are related to the effective rolling radius. In this context, the effective rolling radius is defined as the ratio of the true forward distance traveled by the center of a wheel divided by the angular displacement measured over this distance.
A problem with relying on the effective rolling radius of a radial construction tire is that its radius is only weakly dependent on tire pressure. The large "hoop tension" in the tire belt keeps the tire rolling radius almost constant with respect to tire inflation. For example, tests indicate that a tire inflated to only 5 psi will have a rolling radius approximately 0.9% smaller than if it were inflated to its nominal pressure, 30 psi. However, very accurate measurement of rolling radius has become economically feasible due to the enhanced dynamic range of modern 16-bit microprocessors commonly used in Anti-lock Braking Systems (ABS) and which read the wheel revolution sensors.
Another challenge in detecting low tire pressure is that some tire characteristics have a larger influence upon wheel effective rolling radius than inflation pressure. Tire-to-tire manufacturing tolerances typically vary the effective rolling radius by up to 1.2%. Also, during the tire break-in period, approximately the first 100 miles, effective rolling radius typically can change up to 0.6%. Tread wear also significantly changes the effective rolling radius over the tire lifetime, typically up to 3.6%.
Vehicle operating conditions will also cause significant changes to the effective rolling radius. These conditions are those which cause wheel slippage, those related to the use of a "space-saver" spare tire, and those related to speed. Generally speaking, any maneuver which causes even slight to moderate wheel slippage will cause the effective rolling radius to change by an amount greater than that to be caused by pressure variation alone. Such maneuvers include accelerating, decelerating using brakes, steering through sharp turns, and any combinations of these.
There also are other vehicle operating conditions which influence wheel effective rolling radius in a way not related to tire slippage. These include vehicle operation with a space-saver spare tire, and vehicle operation at very high or very low speeds. The smaller space-saver spare tire has an effective rolling radius that may differ significantly (e.g. 10%) from the other wheels. However, in some performance cars with large diameter brakes, corresponding large diameter wheels, and low profile tires, the use of a space saver spare wheel and tire which is narrow, may have essentially the same effective rolling radius as the other wheels. Differentiating this spare from the other wheels is very difficult in this situation, and the space-saver might be misinterpreted as a low tire.
Vehicle operation at very high speeds, such as those well above the U.S. national legal limit of 65 mph (105 kph), will cause high centrifugal forces in the wheels which can tension the perimeters of the tires in such a way that a low tire will take on the same or larger effective rolling radius as a nominally inflated tire.
Vehicle operation at very low speeds (e.g. less than 6 mph or 10 kph) also poses several problems for effective rolling radius based systems. One is the increased likelihood of wheel slip due to acceleration, deceleration, and steering. This is because low speed operation is not a sustained operating point, but a transitional one during which the car is decelerating to stop, accelerating to normal driving speeds, or steering through sharp turns and corners. Also at low speeds, the wheel rotation sensors' signals drop to a very low amplitude level and become noisy or non-existent. This loss of signal at low speed is a characteristic of current level wheel rotation sensor technology.
In U.S. Pat. No. 5,721,528 (Boesch et al.), assigned to the assignee of the present invention, a system and method was disclosed that monitors four wheel sensors of an automotive vehicle to determined changes in the effective rolling radii of any wheel and a low pressure tire condition.